Remote monitoring of an optical transmission system using line monitoring signals

ABSTRACT

An optical transmission system allows for the remote determination of the output power of each carrier for each repeater. The optical transmission system includes two terminals, an optical path that transmits a plurality of optical signals between the two terminals, and a plurality of repeaters spaced along the optical path. At least one of the terminals generates a first line monitor signal and a second line monitor signal. The second line monitor signal is delayed by a round trip delay from the terminal to a repeater at which the output power is desired to be measured. The terminal then transmits the first line monitor signal on the optical path. Each repeater in the transmission system generates a return line monitor signal in response to receiving the first line monitor signal and transmits the return line monitor signal on the optical path.

FIELD OF THE INVENTION

The present invention is directed to remote monitoring of an opticaltransmission system. More particularly, the present invention isdirected to remote monitoring of an optical transmission system usingline monitoring signals.

BACKGROUND OF THE INVENTION

Long distance optical transmission systems generally require a pluralityof amplifiers located along the length of the optical fibers toperiodically amplify the optical signals. It is essential in thesesystems to provide the ability to remotely monitor the performance ofany amplifier, and to locate the source of system degradation or faultto a particular amplifier or fiber section.

Most known methods for remotely monitoring the performance of opticalamplifiers in an optical transmission system require an optical loopbackpath between adjacent amplifiers on the forward and return opticalpaths, and the generation of a test signal on at least one end of thesystem. For example, U.S. Pat. No. 5,436,746 discloses an opticaltransmission system that includes multiple loopbacks. A test signal isgenerated at the local station, or terminal, and transmitted on aforward path. The test signal is returned to the local station via theoptical loopbacks and a return path. Measurement of the test signalprovides information that is related to the performance of theamplifiers within the optical transmission system.

The method of using loopback paths to remotely measure the performanceof amplifiers has several disadvantages. Specifically, the loopbackmethod requires test signals to be both transmitted and received onassociated fiber pairs at a terminal of the transmission system. Thetest signals must travel over an optical fiber pair (i.e., the forwardpath and the return path). Therefore, the loop loss information providedby the loopback method is ambiguous because there is no way to tell howthe loop losses are distributed between the forward and return path.

Further, the loop loss information provided by the loopback method isredundant because the same information is measured at both terminals ofthe transmission system. In addition, the optical loopback paths betweenadjacent amplifiers cause a significant transmission impairment in theform of crosstalk or added noise. Finally, the loopback method, whenused to provide information in-service (i.e., while the opticaltransmission system is transmitting signals) requires a long time(approximately 2-8 hours) to obtain a measurement due to the typicalpoor signal-to-noise (S/N) ratio of the monitoring signal. Transmissionsystems that utilize multiple carrier wavelengths, and theircorresponding monitoring signal, have lower S/N ratios than singlewavelength systems, and therefore obtaining measurements using theloopback method in these systems impose an even greater time delay.

Based on the foregoing, there is a need for a method and apparatus forremotely measuring amplifier performance that provides measurementinformation quicker and more accurately than known methods, especiallywhen multiple carrier wavelengths are used.

SUMMARY OF THE INVENTION

The above-described needs are met by the present invention whichremotely determines the output power of each carrier for each repeaterin an optical transmission system. In one embodiment, the opticaltransmission system includes two terminals, an optical path thattransmits a plurality of optical signals between the two terminals, anda plurality of repeaters spaced along the optical path.

At least one of the terminals generates a first line monitor signal anda second line monitor signal. The second line monitor signal is delayedby a round trip delay from the terminal to a repeater at which theoutput power is desired to be measured. The terminal then transmits thefirst line monitor signal on the optical path. Each repeater in thetransmission system generates a return line monitor signal in responseto receiving the first line monitor signal and transmits the return linemonitor signal on the optical path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an optical transmission system inaccordance with one embodiment of the present invention.

FIG. 2 is a detailed illustration of a terminal in accordance with oneembodiment of the present invention.

FIG. 3 is a detailed illustration of a repeater in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides a measurement of the output power of eachoptical carrier for each repeater in an optical transmission system.FIG. 1 illustrates an optical transmission system in accordance with oneembodiment of the present invention. The transmission system of FIG. 1is a long distance underwater system that transmits opticalcommunication signals between a terminal 2 and a terminal 3. Thecommunication signals are sent from terminal 2 to terminal 3 via opticalpath 4 and are sent from terminal 3 to terminal 2 via optical path 5.Each optical path 4, 5 includes up to four optical fibers.

A plurality of repeaters 10-13 are located in a series of predeterminedintervals along optical paths 4 and 5. Repeaters 10-13 include anamplifier for each optical fiber passing through them. The amplifiersamplify the optical signals as they travel between terminals 2 and 3.

Terminal 2 includes a constant DC current source 6 which produces aconstant electric current of predetermined magnitude. The currentprovides power to repeaters 10-13 via center conductor 30. Centerconductor 30 is connected to voltage source 7 in terminal 3. Bothcurrent source 6 and voltage source 7 are connected to ground.

FIG. 2 is a detailed illustration of terminal 2 in accordance with oneembodiment of the present invention. Terminal 2 transmits and receivescarrier signals in the form of optical signals in a known manner. Inaddition, terminal 2 transmits and receives line monitoring signals thatare used to measure the power of each carrier of each repeater 10-13 inthe optical transmission system.

A plurality of transmit lasers 151-153 generate optical signals atcarrier wavelengths “1” to “n”. Each transmit laser 151-153 is input toa modulator 154-156. The data desired to be transmitted is also input tomodulators 154-156. The signals output from modulators 154-156 arecoupled to a combiner 150 where they are combined. The resultant signalis then amplified by transmit amplifier 157 and transmitted on one ofthe optical fibers in optical path 4.

In addition, in terminal 2 a line monitoring signal is generated andtransmitted for each carrier of each repeater that is desired to bemeasured. In one embodiment, the line monitoring signal is generated bya line monitoring generator circuit. The line monitoring generatorcircuit generates the line monitoring signal by biphase modulating in amodulator 102 a 150 kHz continuous wave (“CW”) sine wave carrier that isgenerated by an oscillator 200 with a 10 kHz binary pseudo random(“PRN”) sequence that is generated by a PRN sequence generator 104.Simultaneously, a 250 kHz CW sine wave carrier that is generated by anoscillator 203 is biphase modulated in a modulator 108 by a replica ofPRN sequence 104 and delayed in a delay device 106 by a round trip delayto the repeater desired to be measured. The resulting signal frommodulator 108 is output to a 250 kHz bandpass amplifier 202 and thenoutput on line 201 to a line monitor signal receive circuit that isdescribed below.

In other embodiments of the present invention, a line monitoring signalcan be transmitted and received in terminal 3, or in both terminal 2 andterminal 3.

To measure the power of an individual carrier at a specified repeater, achannel select switch 170 is set to connect the generated line monitorsignal via path 172 to the bias input of the transmit laser 151-153 ofthe desired carrier to be measured. This causes the line monitor signalto be impressed as a low-level amplitude modulation (“AM”) signal on thecarrier that is transmitted on optical path 4. In one embodiment, thelevel of the AM signal is approximately one percent of the level of thecarrier signal. The optical carrier containing the line monitor signalis combined with other carriers in combiner 150, amplified in opticalamplifier 157 and transmitted to repeaters 10-13 via one of the opticalfibers in optical path 4.

FIG. 3 is a detailed illustration of repeater 10 in accordance with oneembodiment of the present invention. Repeaters 11-13 are identical torepeater 10.

In repeater 10, optical fibers 32 and 33, which form optical path 5shown in FIG. 1, transmit optical signals from right to left in FIG. 2.Optical fibers 34 and 35, which form optical path 4, transmit opticalsignals in a direction opposite to that of optical fibers 32 and 33.

A pump manifold 50 includes a plurality of pump lasers. The pump lasersare powered by current received from a pump bias control circuit 68 onpath 100. The pump lasers produce pumping power proportional to theinput current in a known manner. The pumping power is output from pumpmanifold 50 on paths 52.

Optical fibers 32-35 each include identical amplifier components.Referring to optical fiber 32, path 52 is coupled to a directionalwavelength selective coupler 40. Coupler 40 causes the optical energyoutput by pump manifold 50 on path 52 to be directed into an erbiumdoped fiber 42 which amplifies optical signals on optical fiber 32.Optical fiber 32 also includes an optical isolator 44 which preventspower from flowing backwards.

Repeater 10 also includes a signal insertion circuit that inserts areturned line monitor signal onto each amplified optical signal outputfrom the repeater. The returned line monitor signal is generated inresponse to the line monitor signal transmitted from terminal 2. Thepower of the inserted returned line monitor signal is proportional to anindividual carrier power for that repeater.

The returned line monitor signal insertion circuit for repeater 10includes photo detectors 80-83 which are coupled to optical fibers 32,34, 35 and 33, respectively. Photo detectors 80-83 output a currentproportional to their respective input optical power. The input opticalpower includes the line monitor signal transmitted from terminal 2 onone of the outgoing optical fibers coupled to terminal 2. An AC currentis output from one of the photo detectors 80-83 in response to the linemonitor signal.

The outputs of photo detectors 80-83 are input to a summation device 78.The output of summation device 78 is coupled to the input of a bandpassamplifier 76 whose passband corresponds to the spectrum of the linemonitor signal. The output of bandpass amplifier 76, which includes onlythe AC current output from photo detectors 80-83, is coupled to asumming device 66. The other input to summing device 66 is a DC currentvia line 64 which is output from center conductor 30.

The output of summing device 66 is input to pump bias control circuit 68which in turn outputs, via line 100, a DC bias current and an AC currentto pump manifold 50. The output power of pump manifold 50 isproportional to the total input current on line 100. The average outputpower of each amplifier in repeater 10 is proportional to its input pumppower. A returned line monitor signal is therefore impressed as very lowlevel amplitude modulation on each of the amplified optical carriers.The returned line monitor signal has a modulation index proportional tothe received carrier power at the repeater output.

Referring again to FIG. 2, the line monitor receive circuit of terminal2 receives the returned line monitor signals inserted by each repeater10-13. All of the incoming carriers to terminal 2 include a returnedline monitor signal. The incoming carriers are received by terminal 2 onthe fibers that comprise optical path 5, amplified by an opticalamplifier 165 and input to a receive section splitter 160. Each carrieris then filtered by channel filters 161-163 and demultiplexed bydemultiplexer 164.

A sample of the incoming signals on optical path 5 are obtained from adirectional coupler 184. The sample is amplified by an optical amplifier186 and detected by a photo detector 188. The electrical signals fromthe photo detector 188 are input to a bandpass amplifier 190. The outputof the bandpass amplifier 190, which contains a received line monitorsignal, is demodulated in a demodulator 192 with the delayed carrierfrom line 201 that was previously described.

The output of demodulator 192 contains a 100 kHz component proportionalto the power of the selected carrier at the repeater being measured andan AC (noise) component associated with the data spectra of the incomingcarriers that fall into the line monitor signal frequency band. Theoutput of demodulator 192 is input to a 100 kHz receiver 194 whosebandwidth is in the range of 1 to 10 Hz. In one embodiment, the linemonitor signal-to-noise ratio in a 1 Hz bandwidth reaches 20 dB within athree second measuring time. In contrast, prior art remote linemonitoring systems require many hours of measuring time to achievesimilar results.

A control and data storage unit 180 selects which channel and whichrepeater is measured. The channel is selected via line 182 which iscoupled to channel select switch 170. The repeater is selected via line196 which is coupled to delay device 106. The round trip delay of thedesired repeater is input to delay device 106. Control and data storageunit 180 stores the measurements of each channel and each repeater thatis output from receiver 194.

When a line monitor signal for each carrier and each repeater has beensent and received by terminal 2, the measurements stored in control anddata storage unit 180 enable a level profile of power as a function ofwavelength and distance to be created for the optical transmissionsystem.

The present invention provides many improvements over the prior artloopback method. Specifically, signal-to-noise ratio is improved becausethe line monitor signal is returned as a low level amplitude modulationsignal attached to each of the optical carriers rather than an additivesignal that competes with one of the returned carriers. The improvedsignal-to-noise ratio enables the present invention to obtain moreaccurate information in a reduced time. In addition, the transmissiondegradation caused by optical loopbacks through the mechanisms of addednoise introduced by signal path crosstalk is eliminated. Finally,because the line monitor signal is returned on all fibers and allchannels, the need for pairing transmit and receive fibers iseliminated.

The present invention is ideally suited to Wavelength DivisionMultiplexed (“WDM”) systems because the signal-to-noise ratio of thedetected returned line monitor signal increases as the product of thenumber of carriers and the baud rate per carrier.

One embodiment of the present invention is specifically illustratedand/or described herein. However, it will be appreciated thatmodifications and variations of the present invention are covered by theabove teachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

For example, although an underwater long distance optical transmissionsystem is described, the present invention can be implemented on anyoptical transmission system that includes repeaters. Further, otheralternative methods of impressing amplitude modulation on the amplifiedsignals other than the method of pump current modulation that is used inthe described embodiment can also be implemented.

Further, in an alternative embodiment, the bandwidth of amplifier 76 ineach repeater can be reduced to a few Hz by, for example, using acrystal filter. A unique crystal frequency can be assigned to eachrepeater. In this embodiment, the line monitor signal can be a simplesine wave with a frequency that corresponds to that of the repeaterbeing monitored.

What is claimed is:
 1. A method for remotely determining the output power of each optical carrier for any repeater in an optical transmission system that comprises one or more repeaters, a first and second terminal, and optical paths over which are transmitted a plurality of optical carriers between the first and second terminals, said method comprising the steps of: (a) generating a first line monitor signal at the first terminal; (b) generating a second line terminal signal at the first terminal, wherein said second line terminal signal is delayed by a round trip delay from the first terminal to a first repeater; (c) transmitting the first line monitor signal on one of the optical paths; (d) generating a return line monitor signal at each of said repeaters in reponse to receiving the first line monitor signal; (e) transmitting the return line monitor signal on one of the optical paths; (f) receiving the return line monitor signal at the first terminal; and (g) demodulating the return line monitor signal with the second line monitor signal.
 2. The method of claim 1, wherein step (c) comprises the step of impressing the first line monitor signal on one of the plurality of optical carriers.
 3. The method of claim 2, wherein step (c) further comprises the step of amplitude modulating one of the plurality of optical carriers.
 4. The method of claim 1, wherein step (e) comprises the step of impressing the return line monitor signal on one of the plurality of optical carriers.
 5. The method of claim 4, wherein step (e) further comprises the step of amplitude modulating one of the plurality of optical carriers.
 6. An optical transmission system for transmitting optical signals, said system comprising: a first terminal and a second terminal; optical paths coupled to said first and second terminals; and one or more repeaters spaced along said optical paths; wherein said first terminal generates a first line monitor signal and a second line monitor signal and transmits the first line monitor signal on an optical path and wherein said second line monitor signal is delayed by a round trip delay from the first terminal to a first repeater; wherein each of said repeaters generates a return line monitor signal in response to receiving the first line monitor signal and transmits the return line monitor signal on an optical path; and wherein said first terminal receives the return line monitor signal and demodulates the return line monitor signal with the second line monitor signal.
 7. A terminal for an optical transmission system that comprises one or more repeaters and an optical path coupled to said repeaters and said terminal that transmits a plurality of optical carriers, said terminal comprising: a line monitor signal generator circuit coupled to said optical path; and a line monitor signal receive circuit coupled to said optical path and said line monitor signal generator circuit; wherein said line monitor signal generator circuit comprises a first and second modulator; a first oscillator coupled to said first modulator and a second oscillator coupled to said second modulator; a binary pseudo random sequence generator coupled to said first modulator; and a delay device coupled to said binary pseudo random sequence generator and coupled to said second modulator.
 8. The terminal of claim 7, wherein said line monitor signal receive circuit comprises: a photo detector coupled to said second optical path; a third modulator coupled to an output of said photo detector and an output of said second modulator; and a receiver coupled to an output of said third modulator.
 9. The terminal of claim 8, further comprising: a control and data storage unit coupled to said line monitor signal generator circuit and said line monitor signal receive circuit, said control and data storage unit selects the fixed amount of time and one of said plurality of optical carriers and stores an output of said line monitor signal receive circuit. 